Treatment of beaten bleached kraft pulp with cellulase preparations can increase the pulp drainability, and pretreatment of unbeaten pulp with cellulases can improve the pulp beatability. To determine the enzyme(s) responsible for these changes in pulp properties, a bleached hardwood kraft pulp was treated with Trichoderma reesei cellulase (Celluclast(TM)) before and after laboratory beating of pulp samples. By fractionating the Celluclast cellulases and evaluating the effectiveness of enzyme components in changing the pulp freeness, an endoglucanase (EG II) was found to play the key role in both the pulp drainability and beatability changes. In contrast, a cellobiohydrolase (CBH I), a major cellulase protein in the Celluclast cellulases, had no effect on either the pulp drainability or beatability. The drainage increase was closely correlated with the endoglucanase activity (assayed as CMCase) in three different cellulase preparations from T. reesei, Aspergillus niger, and Penicillium funiculosum. On the other hand, the beatability enhancement effected by these enzymes had no correlation with their CMCase activities.